The present invention relates to glass-ceramics which has a high elastic modulus and is suitable for use as a substrate for information recording media, e.g., hard disks and optomagnetic disks, as a substrate for electrical/electronic parts, and as an optical part or a substrate for optical parts, and a process for producing the same. The present invention further relates to a substrate for an information recording medium having high performances with small deflection, utilizing advantages of high elastic modulus and high heat resistance of the glass-ceramics.
Glass-ceramics is generally produced by, for example, maintaining an amorphous glass as a precursor therefor at a temperature of from the glass transition point to about the softening point for a certain period of time. Glass-ceramics combine high surface flatness inherent in glasses, and high mechanical strength and heat resistance improved by crystallization, and has no pores or voids that are difficult to eliminate in conventional ceramics. A further feature of glass-ceramics is that properties such as coefficient of thermal expansion can suitably be regulated according to applications of the glasses.
Due to those properties, glass-ceramics has conventionally been used as heat-resistant tableware and building members and is recently used as substrates for electrical/electronic parts, as optical parts, as substrates for optical parts, or as substrates for information recording media. The term xe2x80x9cinformation recording mediaxe2x80x9d used herein means something in which information is retained in any form. Specifically, an information recording medium is something having a recording layer which partly changes magnetically, physically, chemically, or mechanically by the action of magnetism, light, heat, etc., and retains the changed state permanently or temporarily. Furthermore, the term xe2x80x9csubstrates for information recording mediaxe2x80x9d used herein means, for example, substrates for magnetic disks, optomagnetic disks, compact disks (CD), and the like to be integrated into hard disks for use in computers, digital cameras, digital video recorders, etc. However, the substrates for information recording media should not be construed as being limited to those.
Substrates for information recording media (hereinafter referred to as xe2x80x9csubstratesxe2x80x9d for simplicity) are always required to attain higher information recording density and higher rate of recording or reading. In particular, with the recent trend toward the recording or processing of information as digital data, the above requirements are becoming highly important. In order to realize a substrate of higher information recording density, flatter surface of the substrate must be obtained. In hard disks, for example, higher recording density can be obtained by narrower distance between the magnetic head and the substrate (lower glide height) employed. However, if surface flatness of the substrate was low, lower glide height causes a serious problem that the magnetic head comes into contact with projections exist on the substrate surface and is thus damaged. On the other hand, in order to realize a substrate available on higher rate of information recording or readout, mechanical strength (elastic modulus) of the substrate must be improved. In hard disks, for example, a higher rate of recording or readout can be obtained by employing higher rotational speed of the substrate. However, higher substrate rotational speed results in larger substrate bending and a larger amplitude of substrate vibration, and this leads to a strong fear that the magnetic head hits on the substrate and the head, films on substrate, and information recorded are damaged.
Hitherto, substrates made of an aluminum alloy have generally been used for hard disks. Although such substrates made of an aluminum alloy (hereinafter referred to as xe2x80x9caluminum substratesxe2x80x9d) have merits of inexpensive, good formability, etc., they have drawbacks, for example, that the elastic modulus thereof is insufficient and there is no way to improve the elastic modulus thereof, and the surface flatness thereof cannot be improved beyond a certain level. An elastic modulus (Young""s modulus) of these aluminum substrates is 71 GPa, and it is thought that aluminum substrates are inapplicable to hard disks which are driven at a rotational speed of 10,000 rpm or higher and expected to become the mainstream in the future, due to the low elastic modulus thereof. This is because when rotated at such a high speed, the aluminum substrates bend more and vibrate at larger amplitude to cause a problem therefore the glide height cannot be lowered. In addition, since further size reduction will be required to various information recording devices including hard disks in the future, thinner substrates will also be required. However, aluminum substrates cannot meet the above requirement because a thickness reduction therein results in larger bending.
Substrates made of a glass-ceramics (hereinafter referred to as xe2x80x9ccrystallized substratesxe2x80x9d) are superior to aluminum substrates in flatness and elastic modulus. Despite this, however, conventional crystallized substrates have become unable to meet the recent requirements concerning higher recording densities and higher rates of recording or reading sufficiently. Under these circumstances, crystallized substrates of higher elastic modulus have been proposed. For example, JP-A-8-91873 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a glass-ceramics of major crystalline phase containing nickel spinelloid (solid solution of NiAl2O4 and Ni2SiO4). U.S. Pat. Nos. 5,476,821 and 5,491,116 and International Publication WO 98/22405 describe a devised technique in which the composition of a glass-ceramics is changed to improve the elastic modulus.
Further, JP-A-10-188260 discloses a technique of improving the flatness of a glass-ceramics containing lithium disilicate (Li2O.2SiO2) as a major crystalline phase and cristobalite (SiO2) crystals coexistent therewith by adding manganese (MnO) and chromium (Cr2O3) to the glass in an amount of from 1 to 3 wt %.
However, the prior art techniques have the following problems.
The glass-ceramics disclosed in JP-A-8-91873 contains nickel oxide (NiO) in a large amount. Since nickel is an expensive element, this glass-ceramics is expensive and unsuitable for mass production.
The glass-ceramics described in U.S. Pat. Nos. 5,476,821 and 5,491,116 and International Publication WO 98/22405 has a drawback that devitrification occurs during glass forming even if the glass is not cooled rapidly. Consequently, this prior art glass has high devitrification temperature and is hence difficult to mass-produce in a stable manner. Another drawback of this glass-ceramics is that it is difficult to examine the glass-ceramics for defects such as surface mars and adherent foreign substances because the glass-ceramics is milk-white or hazy.
Further, the glass-ceramics disclosed in JP-A-10-188260 has a drawback that its elastic modulus (Young""s modulus) is low because the major crystalline phase thereof comprises lithium disilicate (Li2O.2SiO2).
The present invention has been achieved in view of the above-described problems of the prior art techniques.
One object of the present invention is to provide a glass-ceramics which has a high elastic modulus, can be produced easily, and is inexpensive.
Another object of the present invention is to provide a process for producing the glass-ceramics.
Still another object of the present invention is to provide at low cost a crystallized substrate which can effectively suppress bending or vibrating.
Further object of the present invention is to provide an information recording medium.
Still further object of the present invention is to provide an information recording device.
The glass-ceramics according to the present invention has a major crystalline phase constituted of crystals containing manganese (Mn).
The process for producing the glass-ceramics according to the present invention comprises utilizing as a raw material a grinding or polishing waste which come from grounding or polishing process of the glass-ceramics or a precursor glass.
Another process for producing the glass-ceramics according to the present invention comprises holding a precursor glass at a certain temperature for a certain period of time (first-stage heat treatment) and then holding the precursor glass at a temperature higher than the first-stage heat treatment temperature for a certain period of time (second-stage heat treatment), wherein the first-stage heat treatment is conducted at a temperature higher than the glass transition temperature (Tg) of the precursor glass by from 25 to 100xc2x0 C. for 30 minutes or longer, and the second-stage heat treatment is conducted at a temperature higher than the glass transition temperature (Tg) of the precursor glass by from 75 to 300xc2x0 C. for 10 minutes or longer.
The substrate for an information recording medium according to the present invention comprises the glass-ceramics.
The information recording medium according to the present invention comprises using the crystallized substrate.
The information recording device according to the present invention contains the information recording medium.
The practical embodiments of the present invention will be described in detail below. Hereinafter, all percents are by mol (mol %) and xe2x80x9celastic modulusxe2x80x9d means the modulus represented by Young""s modulus, unless otherwise indicated.
The present invention has been achieved as a result of intensive investigations on glass-ceramics with respect to compositions, heat treatment conditions, precipitated crystal systems, and elastic modulus. Specifically, it has been found that a glass-ceramics having major crystalline phases constituted of crystals containing manganese have an exceedingly high elastic modulus, and that this glass-ceramics can be produced from a precursor glass of lower liquidus temperature if various ingredients including TiO2 are contained therein in appropriate contents. The present invention has been completed based on those findings.
Because of its major crystalline phase containing manganese, the glass-ceramics has higher elastic modulus and can be produced at low cost. Manganese is an inexpensive element contained in minerals occurring in large quantities, such as huebnerite (MnWO4), manganolangbeinite (K2Mn2 (SO4)3), apjohnite (MnAl2(SO4)4.22H2O), and wad (black ocher, bog manganese, or earthy manganese). Consequently, both a high elastic modulus and inexpensiveness, which are effects of the present invention, are attained together by forming manganese-containing crystals so as to constitute major crystalline phases.
Manganese has been used conventionally as one of colorant for glasses, and manganese-containing glasses show brown to black color depending on the content thereof. Glass-ceramics obtained by crystallization of manganese-containing homogeneous glasses (precursor glasses) also have light blown to black color. It is therefore easy to examine these glass-ceramics for surface defects, e.g., mars and foreign substances.
The term xe2x80x9cmajor crystalline phasesxe2x80x9d as used herein means the crystalline phase of the highest amount among all the crystals simultaneously precipitated through crystallization of precursor glass. The amount of crystalline phases precipitated can be determined by either of the following methods or by other methods:
(1) a method in which the glass-ceramics is examined with an optical or electron microscope or the like and the proportion of each crystalline phase in terms of vol % is calculated from the area thereof; and
(2) a method in which the glass-ceramics is analyzed with an X-ray diffractometer to obtain an X-ray powder diffraction pattern and the proportion of each crystalline phase is calculated from the intensity at the corresponding diffraction peak appearing in the curve.
Although the term xe2x80x9cmajor crystalline phasesxe2x80x9d means a certain crystalline phase present in a largest proportion scientifically, two or more crystalline phases may precipitate in nearly the same amount and it may be hard to tell which one is the major phase. According to the above definition, however, the extension of the present invention is clear even in such cases.
The glass-ceramics of the present invention is a colored composition with high elastic modulus, therefore applications thereof are not particularly limited as long as the properties thereof are effectively utilized. For example, when the glass-ceramics is used as optical parts or substrates for optical parts, there are advantages, for example, that the deformation caused by an external force can be reduced due to its high elastic modulus and that stray light can be effectively suppressed without performing any special treatment due to its color. The glass-ceramics can also be used as high-modulus ceramics fibers for use as a reinforcing member or in composite materials, ceramics tiles as external wall materials for buildings, or highly light-shielding bottles for chemicals, so as to take advantage of the above effects. Since thermal expansion coefficients of the glass-ceramics can be controlled in a wide range, it is utilizable also in optical devices as a member for compensating of deviations caused by temperature changes. In particular, from the standpoint of utilizing its high elastic modulus, the glass-ceramics is suitable for use as a crystallized substrate. This crystallized substrate bends less even when it is spun at a high speed, enables a lower flying height of heads, and contributes to increase recording density and recording/reading rate in information recording devices.
The manganese-containing crystals constituting the major crystalline phases preferably contain at least one element selected from the group consisting of aluminum (Al), silicon (Si) and titanium (Ti), and the major crystalline phase in this case preferably has a Mohs"" hardness of 6 or higher. When the crystals containing at least one of these elements constitute the major crystalline phases, the glass-ceramics has a higher elastic modulus. Examples of such crystalline phases include galaxite (MnAl2O4; Mohs"" hardness: 8), spessartite (Mn3Al2Si3O12; Mohs"" hardness: 7), rhodonite (MnSiO3; Mohs"" hardness: 6.5), tephroite (Mn2SiO4; Mohs"" hardness: 6), pyroxmangite (MnSiO3; Mohs"" hardness: 6), pyrophanite (MnTiO3; Mohs"" hardness: 6), and manganese cordierite (Mn2Al4Si5O18; Mohs"" hardness: 7). It is preferred that one or more of these crystalline phases constitute the major crystalline phase. Since the elastic modulus shows how hard to deform by a uniaxial external stress, it is considered that the more harder crystals are contained, the higher the elastic modulus of the glass-ceramics is. As a result of intensive experiments based on this hypothesis, it has been found that a glass-ceramics containing a precipitated crystalline phase having a Mohs"" hardness of 6 or higher has an elastic modulus of 110 GPa or higher.
The manganese-containing crystals may contain metallic elements or ions other than those enumerated above for the purposes of, e.g., obtaining further high elastic modulus, further fine grains of precipitated crystal and precursor glasses of further easy to produce. Examples of such ingredients include magnesium (Mg), zinc (Zn), zirconium (Zr), cerium (Ce), and ions of these. Where the crystals contain magnesium (Mg) or ions thereof, not only the glass-ceramics has a further high elastic modulus and the precursor glass and the glass-ceramics are more homogenized, but also elastic modulus of remained vitreous parts of glass-ceramics is higher than those without containing Mg. Where the crystals contain zinc (Zn) or ions thereof, the precursor glass and the glass-ceramics have a higher degree of homogeneity. Where the crystals contain zirconium (Zr) or ions thereof, not only fine crystal nuclei generate in a large amount, resulting in a dense crystalline phase, but also elastic modulus of remained vitreous parts of glass-ceramics is higher than those without containing Zr. Where the crystals contain cerium (Ce) or ions thereof, elastic modulus of remained vitreous parts of glass-ceramics is higher than those without containing Ce.
Manganese (Mn) is an essential ingredient for forming the major crystalline phase. The Mn content in the glass-ceramics is preferably from 8 to 55% in terms of MnO. If the Mn content is lower than 8%, temperature to melt the batch becomes unacceptably high, and it is difficult to produce a homogeneous precursor glass, and this leads to maldistribution of crystals in glass-ceramics. In addition, the major crystalline phase is less apt to be constituted by crystals without containing manganese and, as a result, elastic modulus of the glass-ceramics becomes lower and the color thereof becomes lighter which brings difficulty in examining the surface defects. The term xe2x80x9cbatchxe2x80x9d as used herein means a mixture of raw glass materials which has been prepared so that each ingredient is contained in a proper content. On the other hand, if the content of manganese exceeds 55%, liquidus temperature of the melt becomes so high that it is difficult to produce a homogeneous precursor glass, and it is hard to obtain a glass-ceramics in which crystals are uniformly distributed. In order to avoid these problems effectively, the content of manganese is preferably from 9 to 50%, more preferably from 9 to 40%, most preferably from 16 to 40%. Where the content of MnO is lower than 16%, the glass-ceramics preferably contains at least 1% MnO because elastic modulus tends to become lower in the glass-ceramics containing such a low MnO content.
SiO2 is an essential ingredient because it is the network former in glass structure and can constitute the framework of crystalline phase. Lower and upper limits of SiO2 content are determined from the viewpoint whether a homogeneous precursor glass can be formed and whether manganese-containing crystals constitute a major crystalline phase or not, respectively. Specifically, the content of SiO2 in the glass-ceramics is preferably from 30 to 75%, more preferably from 33 to 60%, most preferably from 33 to 55%. If the content of SiO2 is lower than 30%, liquidus temperature of the melt becomes unacceptably high and viscosity of the melt is an exceedingly low, therefore the melt devitrifies quickly and it is difficult to form a homogeneous precursor glass. On the other hand, if the content of SiO2 exceeds 75%, viscosity of the melt is an exceedingly high besides the problem of high liquidus temperature, it is difficult to obtain a homogeneous precursor glass. In addition, such too high SiO2 contents result in tridymite (SiO2) or mullite (Al6Si4O13) precipitation as the major crystalline phase, and lower elastic modulus in resultant glass-ceramics.
Al2O3 is an ingredient which constitutes a precursor glass and a crystalline phase. The content of Al2O3 is preferably from 4 to 33%, more preferably from 4 to 27%, most preferably from 5 to 15%. If the content of Al2O3 is lower than 4%, liquidus temperature of the melt becomes so high that it is difficult to form a homogeneous precursor glass and a glass-ceramics of fine grain. On the other hand, if the content of Al2O3 exceeds 33%, part of the Al2O3 remains unmelted in melting process, so it is difficult to form a homogenous precursor glass. Besides the problem of quick rising of liquidus temperature, there also is a problem that the precursor glass containing such too much Al2O3 gives mullite (Al6Si4O13) precipitation as the major crystalline phase and low elastic modulus in resultant glass-ceramics.
MgO is not only very effective to homogenize the precursor glass and the glass-ceramics but also quite a good ingredient to improve the elastic modulus. However, too much MgO content gives rise to undesirable high liquidus temperature of the precursor glass. MgO is hence an optional ingredient. The content of MgO is determined by trade-off between the improvement in elastic modulus and the impairment in liquidus temperature, hence it is preferably from 0 to 20%, more preferably from 0 to 10%. When the content of MnO is lower than 16%, at least 1% of MgO is preferably contained in the glass-ceramics to keep its high elastic modulus of glass-ceramics. When the glass-ceramics contains MgO, MgO can be contained in the major crystalline phase. In this case, elastic modulus of the glass-ceramics becomes higher than that without containing MgO.
ZnO, which is an optional ingredient, is effective to homogenize the precursor glass and the glass-ceramics. The content of ZnO is preferably from 0 to 20%, more preferably from 0 to 12%, most preferably from 0 to 10%. If the content of ZnO exceeds 20%, there is a strong fear of devitrification of the precursor glass. When the major crystalline phase contains ZnO, elastic modulus of the glass-ceramics is slightly decreased although this decrease is negligible.
The content of the bivalent metal oxides (RO=MnO+MgO+ZnO) considerably influences the liquidus temperature of the precursor glass and the elastic modulus of the glass-ceramics. If the content of RO is lower than 25%, elastic modulus of the glass-ceramics is evidently decreased. On the other hand, if the content of RO exceeds 50%, liquidus temperature of the precursor glass is undesirably high, that is, the molten glass tends to be devitrified during cooling/solidification and homogeneous precursor glass is unavailable. Consequently, the content of RO is preferably from 25 to 50%.
TiO2, although an optional ingredient, is not only very effective to homogenize the precursor glass and the glass-ceramics but also quite a good nucleating agent to obtain fine crystals. When incorporated in crystals, TiO2 greatly improves the elastic modulus of the glass-ceramics. Even if some TiO2 is not contained in the precipitated crystals, it improves the elastic modulus of the parts other than the crystals. Consequently, TiO2 should be incorporated into the glass-ceramics of present invention rather than the addition of the other optional ingredients. It is well known that milk-white or opaque glasses contains relatively large amount of TiO2, TiO2 has the side-effect of making the glass milk-white or opaque. Consequently, the content of TiO2 is preferably from 0 to 20%, more preferably from 0.5 to 15%, most preferably from 4 to 15%. If the content of TiO2 exceeds 20%, liquidus temperature of the precursor glass becomes unacceptably high and easier to devitrify and easier to become milk-white.
ZrO2, which is an optional ingredient, is an ingredient well known as a nucleating agent useful for the formation of crystal nuclei. ZrO2 is also effective to improve the elastic modulus of those parts other than crystals in the glass-ceramics. However, if an excessively large amount of ZrO2 is contained in the precursor glass, there is strong possibility of causing unnecessary crystallization (devitrification). The excess addition of ZrO2 is not only harmful to the devitrification resistance of the precursor glass, but also a cause of milk-white glass due to phase separation. Consequently, the content of ZrO2 is preferably from 0 to 10%, more preferably from 0 to 5%, and most preferably from 0.5 to 5%.
CeO2, which is an optional ingredient, serves to improve the elastic modulus of those parts other than crystals in the glass-ceramics. It is well known that CeO2 is also used as a major component of general abrasive materials. The content of CeO2 in the glass-ceramics is preferably 5% or lower, and more preferably from 0 to 3%. If the content of CeO2 exceeds 5%, liquidus temperature of the precursor glass becomes undesirably high and easier to devitrify.
For the purposes of refining during melting, regulating the viscosity of the molten glass, regulating the liquidus temperature, etc., ingredients other than the major ingredients described above can further be added. (Hereinafter, such optional ingredients are referred to as xe2x80x9cminor ingredientsxe2x80x9d.) Examples of the minor ingredients include As2O3, Sb2O3, SO3, SnO2, Fe2O3, CoO, Cl, F, R2O (R is Li, Na, K or Cs) and Rxe2x80x2O (Rxe2x80x2 is Ca, Sr or Ba). Of those minor ingredients, Li2O and Na2O are preferred because they are effective to slightly lower the liquidus temperature, and K2O is preferred because it is effective to increase the viscosity of the molten glass. The upper limit of the content of the minor ingredients is 5%. If the content of minor ingredients exceeds the upper limit, the glass is very easy to devitrify. Besides being purposely added, such minor ingredients, in some cases, come into the glass as impurities of raw materials for major ingredients. The content of the minor ingredients in the glass-ceramics is preferably 8% or lower, more preferably 5% or lower. If the content of minor ingredients exceeds 8%, not only the minor ingredients exert a considerable influence on the contents of major ingredients, but also the glass-ceramics comes to show properties attributable to the minor ingredients. On the other hand, there often are cases where minor ingredients come into the glass in an amount of up to about 2% as impurities of raw materials for major ingredients.
The glass-ceramics in which the contents of the components are within the respective ranges specified above has an elastic modulus of 110 GPa or higher, and is produced from a precursor glass having a liquidus temperature of 1,260xc2x0 C. or lower. The elastic modulus of this glass-ceramics is at least 150% of that of aluminum substrates, which is 71 GPa. As will be demonstrated in Examples given later, the elastic modulus of the glass-ceramics can be reached up to 120 GPa or higher, or can be reached up to 130 GPa or higher, by further regulating the composition within the above range. This glass-ceramics, therefore, is effectively utilizable in various applications where high-modulus characteristics are required. Furthermore, since the liquidus temperature of the precursor glass is 1,260xc2x0 C. or lower or can be 1,100xc2x0 C. or lower, this precursor glass can be produced by the float process which is a general process in industrial production of sheet glass. As long as the precursor glass has a liquidus temperature within that range, the temperature of the molten glass can easily be managed and the furnace can effectively be prevented from being thermally damaged.
The glass-ceramics has superior heat resistance, with the temperature at which the glass-ceramics cannot withstand its own weight (yield point) being 900xc2x0 C. or higher. The glass-ceramics is hence kept away from problems such as deformation even in high-temperature processing. Consequently, if the glass-ceramics is used as a substrate, functional films of higher performances can easily be formed on a surface thereof. When the crystallized substrate is sufficiently heated in forming thereon such functional films, which usually are deposited by sputtering, then the surface texture of the substrate becomes finer to enable a higher density of information recording. The same effect is produced by sufficiently heating the crystallized substrate after the formation of functional films. Namely, the high heat resistance of the glass-ceramics enables an information recording medium to attain a higher information recording density.
Processes for producing the glass-ceramics are not particularly limited and conventional techniques can be used as they are. Examples thereof include the following two processes:
(1) a process comprising a one-stage heat treatment in which a precursor glass is held at a temperature higher than the glass transition temperature thereof by at least 20xc2x0 C. for at least a certain period of time; and
(2) a process comprising a two-stage heat treatment in which a precursor glass is held at a temperature around the glass transition temperature thereof for a certain period of time to generate crystal nuclei and then held at a higher temperature for a period of time sufficient to enable the nuclei to grow.
However, the following process facilitates the formation of a glass-ceramics of higher elastic modulus with a large amount of exceedingly fine crystals precipitated in thereof. This process is a two-sage heat treatment which comprises holding a precursor glass at a temperature higher than the glass transition temperature (Tg) thereof by from 25 to 100xc2x0 C. for 30 minutes or more (first-stage heat treatment) and then keeping the precursor glass at a temperature higher than the Tg by from 75 to 300xc2x0 C. for 10 minutes or more (second-stage heat treatment). The first-stage heat treatment is a conventional step for generating crystal nuclei in the precursor glass. If the temperature for this treatment is below the temperature higher than the Tg by 25xc2x0 C. or if the maintaining time is shorter than 30 minutes, the generation of crystal nuclei is insufficient and, hence, precipitated crystalline phases during the second-stage heat treatment has less contribution to improvement of elastic modulus or precipitated crystals in the second-stage treatment become too large to obtain glass-ceramics of fine crystals. On the other hand, if the temperature for the first-stage treatment is higher than the Tg by more than 100xc2x0 C., there is a high possibility that the precursor glass may deform badly or melt during the first-stage heat treatment. As conventionally known, the second-stage heat treatment is a step for growing the crystal nuclei which have been generated in the precursor glass. However, if the temperature for this treatment is below the temperature higher than the Tg by 75xc2x0 C. or if the holding time is shorter than 10 minutes, the growth of crystals is insufficient and this results in lower elastic modulus in a glass-ceramics. If the temperature for the second-stage heat treatment is not higher than that for the first-stage heat treatment, this second-state treatment is meaningless. On the other hand, if the temperature for the second-stage heat treatment is higher than the Tg by more than 300xc2x0 C., there is a high possibility that precipitated crystalline phases have less contribution to improvement of elastic modulus or the precursor glass may deform badly or melt during the heat treatment.
The precursor glass and the glass-ceramics are processed appropriately according to applications. For example, where the precursor or glass-ceramics is processed into a substrate, a large amount of a polishing waste comes from the polishing process. This polishing waste is a mixture of the glass-ceramics or the precursor glass therefor with an abrasive material. When the main component of abrasive material is comprised CeO2, the polishing waste can hence be a part of raw material for a precursor glass. Consequently, to use this polishing waste as a part of a batch produces new effects, i.e., effective utilization of resources by recycling and environmental protection by the reduction of the discharge amount of the polishing waste as an industrial waste. Although the abrasive material is not particularly limited in components thereof, it preferably comprises one or more of the major ingredients of the glass-ceramics from the standpoint of producing the above effects.
For processing the glass-ceramics into a substrate, conventional techniques can be used as they are. Because of its high elastic modulus, this crystallized substrate has excellent properties such that it is stiff to bend and easy to escape from resonant vibration even when it is thin. Furthermore, since the crystallized substrate itself has a dark color, defects present thereon such as, e.g., mars and foreign substances can be detected easily and precisely even when these defects are microscopic ones. In addition, when a surface of the crystallized substrate is processed by a laser beam, finer processing is possible because the substrate has a large coefficient of heat absorption. Therefore, an information recording device containing this crystallized substrate integrated thereinto can sufficiently meet the requirements concerning higher recording densities, higher recording/reading rates, and smaller thickness, which will become severer in the future.